The present invention relates to a method for producing lactic acid based polyurethane. In addition, the invention relates to certain applications of the polyurethane thus produced.
Lactic acid polymers have been produced by polymerizing lactide, which is a cyclic dimer of the lactic acid. The polylactic acid thus obtained has a high molecular weight and good physical properties. However, drawbacks have been technical difficulties in manufacturing lactide and as a result thereof, the poor total yield of the process. It is also possible to polymerize monomeric acid, but the molecular weight of the product called lactic acid--oligomer thus obtained is so low, generally below 10,000, that the product is not appropriate as a plastic in such. The molecular weight of the oligomer has been increased by making it to react with diol and by copolymerizing it thereafter with isocyanate into polyurethane. Polyurethane based on lactic acid has also been prepared using an equivalent method, starting from lactide, i.e. dimeric acid.
The lactic acid polymers are characterized in that they are biodegradable. For this reason, they have been used for medical applications, such as surgical applications, requiring high quality and purity of the product.
Production methods of lactic acid-based polyurethanes have been described in some prior patent specifications. In the U.S. Pat. No. 4,804,691 a method is disclosed, in which lactide is used as the starting material. The product thus obtained is intended for joining of soft live tissues. In U.S. Pat. No. 1,016,314 and JP application publication No. 63,278,924, production methods of self-degradable polyurethanes appropriate for medical purposes, such as surgical purposes are described, in which a polymer obtained from lactic acid and diol, such as ethylene glycol, is copolymerized further with isocyanate. In JP application specification No. 01,297,420 an equivalent method is disclosed, a product obtained which is described to be resistive and appropriate for a coating material. In JP application publication No. 55,054,322 a polyurethane based on lactic acid and a plurality of other starting materials is furthermore known, said polyurethane being, according to said publication, among other things, resistive to solvents.